On Fri, Jan 18, 2019 at 10:29 PM Jonathan Wakely <jwak...@redhat.com> wrote: > > The old version of _Sp_counted_ptr_inplace::_M_get_deleter (up to GCC > 8.2.0) expects to be passed a real std::typeinfo object, so mixing that > with the new definition of the __shared_ptr constructor (which always > passes the fake tag) leads to accessing the fake object as a real > std::typeinfo. Instead of trying to make it safe to mix the old and new > definitions, just stop using that function. By passing a reference to > __shared_ptr::_M_ptr to the __shared_count constructor it can be set > directly, without needing to obtain the pointer via the _M_get_deleter > back-channel. This avoids a virtual dispatch (which fixes PR 87514). > > This means that code built against new libstdc++ headers doesn't use > _M_get_deleter at all, and so make_shared works the same whether RTTI is > enabled or not. > > Also change _M_get_deleter so that it checks for a real type_info object > even when RTTI is disabled, by calling a library function. Unless > libstdc++ itself is built without RTTI that library function will be > able to test if it's the right type_info. This means the new definition > of _M_get_deleter can handle both the fake type_info tag and a real > type_info object, even if built without RTTI. > > If linking to objects built against older versions of libstdc++ then if > all objects use -frtti or all use -fno-rtti, then the caller of > _M_get_deleter and the definition of _M_get_deleter will be consistent > and it will work. If mixing -frtti with -fno-rtti it can still fail if > the linker picks an old definition of _M_get_deleter and an old > __shared_ptr constructor that are incompatible. In that some or all > objects might need to be recompiled.
Can you try summarizing whatever caveats result from this in the 8.3/9 changes.html parts? I have a hard time extracting that myself from the above ;) (small example what kind of simplest code might run into this helps) Richard. > PR libstdc++/87514 > PR libstdc++/87520 > PR libstdc++/88782 > * config/abi/pre/gnu.ver (GLIBCXX_3.4.26): Export new symbol. > * include/bits/shared_ptr.h > (shared_ptr(_Sp_make_shared_tag, const Alloc&, Args&&...)) > (allocate_shared): Change to use new tag type. > * include/bits/shared_ptr_base.h (_Sp_make_shared_tag::_S_eq): > Declare new member function. > (_Sp_alloc_shared_tag): Define new type. > (_Sp_counted_ptr_inplace): Declare __shared_count<_Lp> as a friend. > (_Sp_counted_ptr_inplace::_M_get_deleter) [!__cpp_rtti]: Use > _Sp_make_shared_tag::_S_eq to check type_info. > (__shared_count(Ptr, Deleter),__shared_count(Ptr, Deleter, Alloc)): > Constrain to prevent being called with _Sp_alloc_shared_tag. > (__shared_count(_Sp_make_shared_tag, const _Alloc&, Args&&...)): > Replace constructor with ... > (__shared_count(Tp*&, _Sp_alloc_shared_tag<_Alloc>, Args&&...)): Use > reference parameter so address of the new object can be returned to > the caller. Obtain the allocator from the tag type. > (__shared_ptr(_Sp_make_shared_tag, const Alloc&, Args&&...)): Replace > constructor with ... > (__shared_ptr(_Sp_alloc_shared_tag<Alloc>, Args&&...)): Pass _M_ptr > to the __shared_count constructor. > (__allocate_shared): Change to use new tag type. > * src/c++11/shared_ptr.cc (_Sp_make_shared_tag::_S_eq): Define. > > Tested powerpc64le-linux, committed to trunk. > > I'll backport this to gcc-8-branch without the new symbol in the library. > >